1. Field of the Invention
The present invention relates generally to composite sandwich panels which are used in aerospace and other applications where light weight and high strength are required. More particularly, the present invention is directed to composite sandwich panels which are designed for use in an environment where flame resistance and/or fire protection is required.
2. Description of Related Art
Composite sandwich panels are widely used in aerospace and other industries where structures are required that are light weight and strong. The sandwich panels typically include a light weight central core structure which is sandwiched between two composite face sheets. The face sheets are adhesively bonded to the core. Various core structures are presently in use with the two main types being rigid foam and honeycomb. Rigid foam cores are advantageous because the face sheets contact the foam over a relatively large surface area which insures a strong bond. In many foam cores, the core will fracture before the adhesive bond between the face sheets and the core fails. Some disadvantages of rigid foam core are that light weight rigid foam is not particularly strong and the fire or flame resistance of many foams is not particularly good.
Honeycomb provides a number of advantages over rigid foam cores. Honeycomb cores, in general, provide much greater strength than rigid foam cores having the same density. One drawback of a honeycomb core is that the surface area which is available for bonding to the face sheets is much smaller than for foam cores. The honeycomb cells extend transversely between the face sheets so that the only surface available for bonding to the face sheets is the outer edges of the cells. As a result, the overall strength of the majority of honeycomb panels is significantly affected by the strength of the bond between the face sheets and the honeycomb.
Epoxy-based adhesives have been used to bond face sheets to honeycomb cores where the structural strength of the panel is important. For example, honeycomb sandwich panels used for aircraft flooring and other high stress structures have typically utilized epoxy-based adhesives to bond the face sheets to the honeycomb core. Epoxy-based adhesives tend to burn relatively easily and produce large amounts of smoke. Attempts have been made to increase the fire resistance of epoxy adhesives by halogenating the adhesives. However, these attempts have not been entirely successful.
Phenolic adhesives are inherently more fire resistant than epoxy adhesives. However, phenolic resins do not have the same strong adhesive properties found in epoxy resins. As a result, phenolic adhesives have been limited to use in honeycomb sandwich panels which are not structural in nature. Such panels include aircraft interior side walls, ceilings and overhead bins. It was found that certain configurations and combination of phenolic resins and fire protection agents provide low flame, smoke and toxicity (FST) panels which have structural strengths which are equivalent to prior panels using epoxy-based adhesives. This discovery is set forth in PCT Application No. US00/06609 which is owned by the same assignee as the present invention.
In many situations, the flame/heat resistance of a composite panel is of primary importance. Making such flame/heat resistant panels is particularly problematic when the central core is made from low heat resistant materials such as NOMEX® (aramid paper) or cellulose-based resin impregnated fibers. One approach for thermally protecting such cores is to make the face sheets resistant to flame and heat. Exemplary composite panels which include flame/heat protective face sheets are disclosed in U.S. Pat. Nos. 4,557,961; 4,299,872; 4,598,007;5,309,690; and 4,037,751. The face sheets described in these patents utilize an organic polymer resin matrix which includes one or more intumescent materials.
Heat resistant composite panels have also been made where the face sheets utilize an inorganic resin matrix. For example, U.S. Pat. No. 5,798,307 discloses the preparation of a composite panel in which a NOMEX® core is sandwiched between two face sheets composed of carbon fiber fabric impregnated with an alkali alumino-silicate geopolymeric matrix. In the patent disclosure, no separate adhesive is used to bond the face sheets to the core. Instead, as is known in the art, the uncured face sheets are applied directly to the core and cured (hardened) in place. As a result, the geopolymeric matrix acts as the adhesive.